Messages in this thread | | | Date | Thu, 19 Jan 2023 10:43:38 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
| |
On Thu, Jan 19, 2023 at 11:41:01AM -0500, Alan Stern wrote: > On Thu, Jan 19, 2023 at 12:22:50PM +0100, Jonas Oberhauser wrote: > > > > > > On 1/19/2023 3:28 AM, Alan Stern wrote: > > > > This is a permanent error; I've given up. Sorry it didn't > > > work out. > > > > [It seems the e-mail still reached me through the mailing list] > > [For everyone else, Jonas is referring to the fact that the last two > emails I sent to his huaweicloud.com address could not be delivered, so > I copied them off-list to his huawei.com address.] > > > > > I consider that a hack though and don't like it. > > > It _is_ a bit of a hack, but not a huge one. srcu_read_lock() really > > > is a lot like a load, in that it returns a value obtained by reading > > > something from memory (along with some other operations, though, so it > > > isn't a simple straightforward read -- perhaps more like an > > > atomic_inc_return_relaxed). > > The issue I have with this is that it might create accidental ordering. How > > does it behave when you throw fences in the mix? > > I think this isn't going to be a problem. Certainly any real > implementation of scru_read_lock() is going to involve some actual load > operations, so any unintentional ordering caused by fences will also > apply to real executions. Likewise for srcu_read_unlock and store > operations. > > > It really does not work like an increment at all, I think srcu_read_lock() > > only reads the currently active index, but the index is changed by > > srcu_sync. But even that is an implementation detail of sorts. I think the > > best way to think of it would be for srcu_read_lock to just return an > > arbitrary value. > > I think I'll stick to it always returning the initial value. Paul said > that would be okay.
Just confirming.
> > The user can not rely on any kind of "accidental" rfe edges between these > > events for ordering. > > > > Perhaps if you flag any use of these values in address or control > > dependencies, as well as any event which depends on more than one of these > > values, you could prove that it's impossible to contrain the behavior > > through these rfe(and/or co) edges because you can anyways never inspect the > > value returned by the operation (except to pass it into srcu_unlock). > > > > Or you might be able to explicitly eliminate the events everywhere, just > > like you have done for carry-dep in your patch. > > On second thought, I'll make it impossible to read from the > srcu_read_unlock events by removing them from the rf (and rfi/rfe) > relation. Then it won't be necessary to change carry-dep or anything > else.
Although that works very well for srcu_read_lock() and srcu_read_unlock(), it would be an issue for srcu_down_read() and srcu_up_read(). But one thing at a time! ;-)
Thanx, Paul
| |